Continuously produced melt reacted melamine-formaldehyde resins

ABSTRACT

A method for producing a melamine-formaldehyde resin by reacting crystalline melamine and solid formaldehyde in a single pass reactor, the resin produced by said process and a reinforcing substrate coated with said melamine-formaldehyde resin, are disclosed.

BACKGROUND OF THE INVENTION

Amino resins, e.g. melamine-formaldehyde resins have been known and usedcommercially for over half a century. Melamine-formaldehyde resins havehistorically been prepared in aqueous solution in a single stage, batchreaction. Although a majority of the melamine-formaldehyde resins nowcommercially available are produced using batch reactions, continuouslyproduced resin is known. The commercial production of continuouslyproduced resin however, requires cascade feeding through a series ofstirred tank reactors or reaction on a rubber mill, both proceduresrequiring extended periods of time.

SUMMARY OF THE INVENTION

It has now been found that melamine-formaldehyde resins can be producedcontinuously utilizing short periods of reaction so as to result inenergy and labor savings. The resins are produced as 90-100% solidmaterials in the substantial absence of water or any other solvent. Theprocess of the present invention results in the production ofmelamine-formaldehyde resins which exhibit properties materiallydifferent than prior art solution produced resins when coated onreinforcing substrates. The instant process is very rapid as compared toknown batch procedures and results in resins which can be cooled andground into powder or coated onto reinforcing substrates while stillmolten. Utilizing the aqueous batch procedures of the prior art, inorder to achieve a powdered resin, the liquid resin produced must firstbe subjected to a drying step such as spray-drying. Furthermore, if theresin solution produced is used to saturate paper sheets in the mannernormally followed for the production of laminated articles, energy mustbe employed to evaporate the solvent from the sheet after impregnation.Additionally, coreactants and modifiers cannot be added to resinsolutions unless they are co-solvable in the solvent employed.

The instant process allows the incorporation into themelamine-formaldehyde resin of the necessary or desired co-reactants andmodifiers during the resin formation. Furthermore, the resins formed canbe coated directly onto reinforcing substrates for ultimate laminateproduction immediately upon recovery from the reactor as moltenmaterials.

DESCRIPTION OF THE INVENTION INCLUDING PREFERRED EMBODIMENTS

The present invention comprises a continuous method for the one-shotpreparation of a melamine-formaldehyde resin comprising:

(a) charging crystalline melamine and solid formaldehyde in the absenceof solvent and at a ratio of formaldehyde to melamine ranging from about1:1 to about 3:1, respectively, into a single pass continuous reactor ata temperature ranging from about 120° C. to about 180° C. for from about30 seconds to about 5 minutes and

(b) recovering the resultant resin as a molten material.

This process enables the manufacture of melamine-formaldehyde resinsutilizing a continuous process and substantially solvent-free media. Theprocess comprises either blending the reactants, and any coreactants orother modifiers, together and then subjecting them to reaction in acontinuous reactor or feeding each component into the reactorindividually. The crystalline melamine and solid formaldehyde, whenblended, form an essentially 100% reactive composition. They may beblended together in any suitable manner such as by the use of a mixer orother device which insures the formation of a homogeneous distributionthereof. Examples of coreactants include triethanolamine, hydroxypropylmelamine, 1,4-butanediol etc. and examples of modifiers include anythermoplastic material melting within the above range of reactiontemperatures, for example, low molecular weight, low densitypolyethylene, low-melting polyamides, biscarbamates and the like. Otheruseful modifiers include fibers, pigments, abrasives, microbubbles,stabilizers, absorbers, dyes and the like.

The reactive components in the form of the resultant homogeneous blendor individual charges are fed into a continuously stirred, single-passreactor such as a single-screw extruder, a twin-screw extruder, akneader extruder, a series of Moyno pumps, a great extruder i.e. anyreactor that transports a reaction product of increasing viscosity, attemperatures substantially higher than those used to produce resins viaa batch reaction and for residence times of only a fraction of thenormal batch systems. In accordance with the instant invention,temperatures ranging from about 120° C. to about 180°, preferably fromabout 125° C. to about 155° C. and reaction times of from about 30seconds to about 5 minutes, preferably from about 2 minutes to about 3minutes, are employed. Under these conditions, an addition reactionbetween the melamine and the formaldehyde is achieved with a minimum ofany condensation reaction occurring. Depending upon the specificconditions employed, the reaction can be controlled so as to recover aresin of any desired melt viscosity, a possibility not achievable usingknown solution technology.

The novel resins resulting from the process of the present invention maybe recovered from the reactor as a liquid melt or, alternatively, as asolid. They are, as mentioned above, addition reaction resins and aresolid at room temperature. Upon exiting from the reactor, they are ofhigh solids and contain small amounts of water i.e. 2-8%, and usuallysome unreacted charge materials. They are normally 95-100% resin and ina molten state.

Upon recovery from the exit end of the one-pass reactor, themelamine-formaldehyde resins can be cooled to room temperature in anymanner and ground to powdery form utilizing known granulating equipment.

Alternatively, the resins which exit the reactor can be spread, whilemolten, onto a substrate e.g. paper, such as kraft paper, α-cellulosecontaining paper etc., non-woven webs, cloth, glass and the like. Theresultant novel coated substrates can then be cooled and stored and/orused in any desired manner such as for the production of low or highpressure laminates. In the case of paper substrates, the resultantcoated paper is not brittle and can be rolled, a feature contrary topaper which is treated with aqueous solutions of melamine formaldehyderesins produced via batch procedures and then dried. While not wishingto be bound by any particular theory as to why such coated paper sheetsare flexible, it is believed that the high solids and low water contentof the resins produced by the instant process enables them to be coatedonto the paper without destroying the hydrogen bonding of the paper orcausing cellulose swelling and resin penetration which usuallyaccompanies solution impregnation thereof.

The following examples are set forth for purposes of illustration onlyand are not to be construed as limitations on the present inventionexcept as set forth in the appended claims. All parts and percentagesare by weight unless otherwise specified.

EXAMPLE 1

Two moles of flake paraformaldehyde, one mole of melamine crystals and0.05 mole of triethanolamine are blended together in a suitable mixer toform a uniform mixture thereof. This mixture is then fed into aone-pass, single screw extruder operating at a 130° C. barreltemperature and a screw speed controlled to result in a productresidence time in the reactor of approximately three minutes. Theresultant resin is discharged from the reactor and then cooled to asolid mass. It is then mechanically ground. The powdered resin isdissolved in an alcohol-water solution. The solution is then used toimpregnate an α-cellulose containing decor sheet, which sheet is thendried and heat and pressure consolidated into a decorative laminate,using conventional phenolic resin impregnated kraft sheets as the core,which passes all NEMA specifications.

EXAMPLE 2

To a suitable blending apparatus are added and blended 1.6 moles ofsolid paraformaldehyde, 1.0 mole of crystalline melamine, 0.07 mole of1,4-butanediol, 0.04 mole of triethanolamine and 0.08 mole ofhydroxypropylmelamine. The resultant blend is processed using acontinuous, one-shot, one-pass, single screw reactor operating at 150°C. and a residence time of 13/4 minutes. The hot discharge isimmediately coated onto α-cellulose containing decor paper andsolidified by cooling to room temperature. When the paper sheet isemployed as a component in a high pressure decorative laminate assemblyas in Example 1 and heat and pressure consolidated, an excellentdecorative laminate results.

EXAMPLE 3

The procedure of Example 2 is again followed except that about 10% ofthe total ingredients added to the mixing apparatus includes titaniumdioxide powder. The blended materials are fed through the extruder at130° C. with a residence time of two minutes and the continuousdischarge stream is then cooled. The solidified product is broken intolumps and further ground to a fine white powder. The powder is thenmixed with purified wood fibers, air-laid onto a foraminous belt andmolded into a solid reinforced sheet.

EXAMPLE 4

The procedure of Example 3 is again followed except that the hotcontinuous discharge is immediately coated onto a sheet of 60 -cellulosecontaining paper and cooled. The resultant sheet is used as the decorsheet in a high pressure laminate assembly as in Example 1 and heat andpressure consolidated into an excellent white decorative laminate.

EXAMPLE 5

The procedure of Example 3 is again followed except that glassmicroballoons are added to the mixer and processed through the extruder.The resultant composition is then cast into 1/2" thick slabs and formedinto bathroom sinktops.

EXAMPLE 6

The procedure of Example 3 is again followed except that a commerciallyavailable thermoplastic acrylic resin is admixed with the ingredientsbefore extrusion. A moldable product is recovered.

EXAMPLE 7

The procedure of Example 4 is again followed except that the hotcontinuous discharge is coated in a nip between the raw α-cellulosecontaining paper and a texturizing release sheet. The resultant paperassembly is used in the manufacture of a high pressure decorativelaminate between a phenolic resin impregnated kraft paper core assemblyand a polished press plate. An excellent laminate is recovered.

EXAMPLE 8

The procedure of Example 7 is again followed except that the texturingrelease sheet is replaced by an endless release belt. When the paperresulting from this coating is used as the decor sheet in the productionof a low pressure laminate on particleboard, an excellent laminateresults.

EXAMPLE 9

The procedure of Example 7 is again followed except that the endlessbelt is replaced by an overlay sheet. The resultant overlay-decorcomposite is then used as the top layer of a high pressure decorativelaminate, the overlay sheet comprising the topmost layer. A decorativelaminate passing all NEMA standards is recovered.

EXAMPLE 10

The procedure of Example 1 is again followed except that a twin-screwextruder is used. Substantially equivalent results are observed.

EXAMPLE 11

The use of a kneader extruder in lieu of the extruder of Example 1results in the production of an excellent resinous product.

EXAMPLE 12

The procedure of Example 7 is again followed except that the heatcontinuous discharge is coated in a nip between the raw α-cellulosecontaining paper and a decor sheet. An excellent laminate is formed.

We claim:
 1. A continuous method for the one-shot preparation of amelamine-formaldehyde resin which comprises:charging crystallinemelamine and solid formaldehyde, in the absence of solvent, into asingle pass reactor at a temperature ranging from about 120° C. to about180° C. for from about 30 seconds to about 5 minutes and recovering theresultant resin as a molten liquid.
 2. The method of claim 1 wherein theresultant liquid resin is cooled and ground into a powder.
 3. The methodof claim 1 wherein said melamine and said formaldehyde are homogeneouslyadmixed before charging to said reactor.
 4. The melamine-formaldehyderesin produced by the process of claim 1.